This invention relates in general to color signal correction circuits and, in particular, to a color signal correction circuit that is entirely integratable. In the NTSC color television system, as in any color television system, human flesh tones are difficult to produce consistently well. This is due to a variety of well-known factors. Unfortunately, the human eye is very sensitive to errors in flesh tone, and any tendency for flesh tones to shift toward either green or purple is instantly recognizable. Consequently, many circuits for augmenting flesh tone colors have been developed and used. Such circuits generally operate on the basis that flesh tones are primarily located along the +I axis, which essentially encompasses orange tones. Prior art circuits combine portions of the R-Y and B-Y color difference signals to develop signals lying along the I axis. As is well known, since the gamut of flesh tones generally lie along the +I axis, the developed I axis signal is indicative of the amount and amplitude of flesh tone colors in the received video. There are correction circuits in the prior art that enhance or augment signals lying along the +I axis based upon their amplitude. In accordance with conventional terminology, the red, green and blue video signals are referred to as R, G and B, respectively, and the luminance or monochrome signal is referred to as Y. The three color difference signals are thus R-Y, G-Y and B-Y.
In U.S. Pat. No. 4,219,840, issued Aug. 26, 1980, and U.S. Pat. No. 4,285,005, issued Aug. 18, 1981, both in the name of the present inventor, an automatic color corrector circuit enables color correction of chroma signals within a selected range of the flesh tone axis by combining the outputs of the chroma demodulators to develop a control signal in response to which a color modifier automatically reduces the gain of the B-Y demodulator by a predetermined factor. The color modifier also adds a predetermined negative B-Y component of fixed value to the output of the B-Y demodulator for helping to correct flesh tone chroma signals which might otherwise have an unnatural cast. The circuit described may be readily fabricated in integrated form since it does not use any variable resistors or capacitors.
U.S. Pat. No. 4,296,432, issued Oct. 20, 1981, describes and claims a joint invention of the present inventor comprising a dynamic enabling network for turning on and off a color corrector circuit. The network receives and combines selected color difference signals (R-Y and B-Y) that are superimposed on a DC bias level for generating a first control signal, the amplitude of which is near a maximum when the color difference signals are representative of flesh tone hues. A second, DC control signal, also derived from the R-Y and B-Y color difference signals, has an amplitude that is a function of the DC bias level associated with these color difference signals. The DC in these control signals is offset and the color corrector circuit is enabled only when the difference in amplitude between them is indicative of flesh tone hues of a minimum amplitude. That circuit suffers the drawback of utilizing a capacitor in a sample-and-hold circuit for developing the second, DC control signal. It is not easily preferable to integrate the capacitor because it consumes a large area, and consequently, an extra pin is required on the integrated circuit for connecting the capacitor thereto. All of the three above-mentioned patents are hereby incorporated by reference herein.
The present invention eliminates the need for the capacitor in the sample-and-hold circuit by providing a counter and a digital-to-analog (D/A) converter for developing a DC potential as a function of the count in the counter. The color signal correction circuit is utilized in an all-digital television environment and enables maximum utilization of integrated circuitry.